The purpose of this study was to develop a metastatic brain tumor model of breast cancer in the rat to monitor the natural history of the disease using a multimodal imaging approach with magnetic resonance imaging (MRI) scanner and in vivo bioluminescence imaging (BLI). MDA-MB231BR cell line is a brain seeking metastatic breast cancer line that was stably transfected to express the firefly luciferase cDNA (LUC) and was magnetically labeled with superparamagnetic iron oxide nanoparticles (SPION) ex vivo for early monitoring by MRI. Nude rats underwent intracardiac infusion SPION ferumoxides labeled MDAMB231BRLUC (231BRL) cells. FEPro labeled 231BRL cells injected rats developed multiple brain and spinal cord metastasis and all rats with brain lesion had multiple skeletal metastasis. Prussian blue positive breast cancer cells could be detected in animals up to 1 week following intracardiac injection of SPION labeled cells. BLI demonstrated increase in luciferase photon flux activity in the brain and bones of the rats while MRI revealed numerous hypointense regions corresponding to SPION labeled cells in the brain within the first 3 days following injection of cell. Quantifying the number of labeled stem cells in target tissues is of great importance to optimize dose and timing of cellular therapy. We has employed a multi-gradient echo pulse sequence to calculate T2* relaxation times to improve the detection of labeled cells within the voxel. We also have investigated the use of Blood oxygen level dependant (BOLD) MRI has been used to evaluate metastatic tumor oxygenation and perfusion. It is unknown whether BOLD can be used for diagnosis in addition to its prognostic potential. Day 1 post injection of FEPro breast cancer cells, numerous hypointense regions were detected on T2*w images throughout rat brain consistent with clusters of tumor cells in the vasculature. At day 36 post injection of cells a total of 34 cortical tumors were identified on the T2-weighted ex vivo MR images. These regions were mapped to the in vivo images. Of these tumors, 26 had noticeable BOLD decreases in the corresponding region from the week prior (day 29) MRI, and 17 had BOLD decreases 2 weeks prior (day 22) MRI. Importantly, the BOLD effects could be detected when the tumor was less than 500 m. BOLD MRI was an early marker of metastatic tumor development, and the changes are consistent with vascular abnormalities from undiagnosed micrometastasis disease. Despite the difficulties in attributing BOLD changes to oxygenation or perfusion, BOLD MRI may aid the detection of developing metastases. Pharmacological approaches to treat breast cancer metastases in the brain have been met with limited success. In part, the impermeability of the blood brain barrier (BBB) has hindered delivery of chemotherapeutic agents to metastatic tumors in the brain. BBB-permeable chemotherapeutic drugs are being developed, and noninvasively assessing the efficacy of these agents will be important in both preclinical and clinical settings. We used dynamic contrast enhanced (DCE) and diffusion weighted imaging (DWI) are magnetic resonance imaging (MRI) techniques to monitor metastatic breast cancer in the rat brain, tumor vascular permeability and cellularity. We demonstrate that brain and bone metastases in the MB 231BR model develop with distinct physiological characteristics as measured with MRI. Brain metastases had limited permeability of the BBB as assessed with DCE and an increased apparent diffusion coefficient (ADC) measured with DWI compared to the surrounding brain. Microscopically, we showed that brain metastases were highly infiltrative, grew through vessel co-option, and caused extensive edema and injury to the surrounding neurons and their dendrites. By comparison, metastases situated in the leptomenengies or in the bone were demonstrated to have high vascular permeability and significantly lower ADC values suggestive of hypercellularity. On histological examination, tumors in the bone and leptomenengies were solid masses with distinct tumor margins. The different characteristics of these tissue sites highlighted the influence of the microenvironment on metastatic tumor growth. The implication of this work raises the issue of the suitability of DWI and DCE to evaluate the response of chemotherapeutic and anti-angiogenic agents used to treat co-opted brain metastases in contrast to these techniques being used in solid tumor masses